1. Field of the Invention
The present invention relates to a novel maleimide copolymer which is excellent in heat resistance, transparency, chemical resistance, impact resistance and moldability, and to a process for producing the same.
2. Description of Related Art
The applications of transparent molding materials have been widened recently. For example, there are exemplified optical information storage carrier of a videodisk, an audiodisk, etc., lenses of a camera, an optical pickup, etc., a cover of a headlight of a car and the like. High heat resistance and high impact resistance have been required for these applications.
A polycarbonate resin has been heretofore used as a transparent heat-resistant molding material. The polycarbonate resin is a transparent resin excellent in heat resistance and impact resistance. For the above-mentioned applications, however, the polycarbonate resin has a drawback in that it is relatively expensive and inferior in chemical resistance and moldability. The applications thereof is, therefore, limited.
Various copolymers of maleimide monomers and other vinyl compound monomers have been proposed as resins which are excellent in heat resistance (Japanese Patent Application Kokai No. 58-162616, Japanese Patent Application Kokai No. 61-276807 and Japanese Patent Application Kokai No. 60-79019). The maleimide copolymers proposed in these patent applications are not, however, satisfactory for the above-mentioned uses. Particularly, there are few reports on the improvement of coloration (decrease in yellowing), which is essential for transparent molding materials. Emulsion polymerization or suspension polymerization is often conducted in the polymerization of a maleimide monomer. Since, however, an emulsifier and the other auxiliaries cause considerable coloration, the emulsion and suspension polymerization are not suitable for the production of transparent molding materials.
In order to improve properties such as transparency and the like, bulk polymerization or solution polymerization is preferably conducted. Since a maleimide monomer, however, exhibits a very high rate of polymerization and therefore is easy to polymerize by itself, the obtained copolymer results in a wide molecular weight distribution, a wide composition distribution and inferior impact resistance.
The maleimide copolymer of the present invention has as high heat resistance as polycarbonate resins. For that purpose, the particular range of the amount of a maleimide monomer is prescribed. When the amount is over the range, the obtained copolymer is inferior in impact resistance and fluidity. The fed amount of the other vinyl compound monomers used in the present invention are also limited to the particular range.
When an unreacted maleimide monomer exists in the obtained copolymer, the copolymer is inferior in transparency and is colored considerably.
Therefore, polymerization methods and the removal of unreacted monomers and organic solvents must be investigated eagerly.